The present invention relates generally to apparatus for performing operations in subterranean wells. More specifically, the invention relates to a seal assembly for dual string coil tubing injection into wells, and a method of preventing fluid leakage during injection of two coil tubing strings into wells for certain downhole operations.
Continuous reeled pipe, generally known within the energy industry as coil tubing string, has been used for many years. It is much faster to run into and out of a well casing than conventional jointed tubing.
Typically the coil tubing string is inserted into the wellhead through a lubricator assembly or a stuffing box because there is a pressure differential between the well bore and atmosphere. The pressure differential may have been naturally or artificially created and serves to produce oil or gas, or a mixture thereof, from the pressurized well.
The coil tubing strings are run into and out of well bores using coil tubing string injectors, which force the coil tubing strings into the wells through a lubricator assembly or stuffing box to overcome the well pressure until the weight of the coil tubing string exceeds the force applied by the well pressure that acts against the cross-sectional area of the coil tubing string. However, once the weight of the coil tubing string overcomes the pressure, the coil tubing string must be supported by the injector. The process is reversed as the coil tubing string is removed from the well.
A method for running dual jointed tubing strings into and out of wells is described in U.S. Pat. No. 4,474,236, entitled METHOD AND APPARATUS FOR REMOTE INSTALLATION OF DUAL TUBING STRINGS IN A SUBSEA WELL which issued to Kellett on Oct. 2, 1984. Kellett describes a method and apparatus for completing a well using jointed production and service strings of different diameters. The method includes steps of running the production string on a main tubing string hanger while maintaining control with a variable bore blowout preventer, and then running the service string into the main tubing string hanger while maintaining control with a dual bore blowout preventer, with the two jointed tubing strings oriented thereto.
The use of coiled tubing for various well treatment processes such as fracturing, acidizing and gravel packing is well known. Typically, several thousand feet of flexible, seamless tubing is coiled onto a large reel that is mounted on a truck or skid. A coiled tubing injector with a chain-track drive, or some equivalent, is mounted above the wellhead and the coiled tubing is fed to the injector for injection into the well. The coil tubing string is straightened as it is removed from the reel by a coil tubing guide that aligns the coiled tubing string with the well bore and the injector mechanism.
Although the use of dual string coil tubing for well servicing and production is known, the prior art fails to teach a method or apparatus for injecting two coil tubing strings into a well bore at the same time. Recent developments in well completion and well workover have, however, demonstrated the utility of using two coil tubing strings concurrently for many downhole operations. The difficulty with injecting dual string coil tubing into a well bore is the proximity of the respective coil tubing strings and the consequent lack of working space to deploy a pair of prior art coil tubing string injector assemblies mounted above the wellhead. This problem is solved by the Applicant with a coil tubing string injector assembly adapted to simultaneously inject dual string coil tubing into a well bore, as disclosed in the Applicant""s copending U.S. patent application Ser. No. 09/779,087 entitled DUAL STRING COIL TUBING INJECTOR ASSEMBLY which is filed concurrently herewith and incorporated herein by reference.
Another problem associated with the injection of dual string coil tubing into a well bore is the prevention of fluid leakage during the injection of the dual string coil tubing, especially when a long downhole tool is connected to one or both of the coil tubing strings. Downhole tools typically have a larger diameter than the coil tubing string, and cannot be plastically deformed, which presents certain difficulties. It is known in the art to overcome these difficulties while injecting a single coil tubing string. For example, U.S. Pat. No. 4,940,095, entitled DEPLOYMENT/RETRIEVAL METHOD AND APPARATUS FOR WELL TOOLS USED WITH COILED TUBING, which issued to Newman on Jul. 10, 1990, discloses a method of inserting a well service tool connected to a coiled tubing string, which avoids the high and/or remote mounting of a heavy coiled tubing injector drive mechanism. A closed-end lubricator is used to house the tool until it is run down through a blowout preventer connected to a top of the well. The pipe rams of the blowout preventer are closed around the tool to support it while a tubing injector is mounted to the wellhead and the coil tubing string is connected to the tool. The blowout preventer is then opened and the coil tubing string injector is used to run the tool into the well. Newman fails to address the use of dual string coil tubings, however.
There is therefore a need for an apparatus and method for prevention of fluid leakage during the injection of dual string coil tubing into a well bore.
It is one object of the present invention to provide a seal assembly for dual string coil tubing injection into a well bore.
It is another object of the invention to provide a method for prevention of fluid leakage during the injection of dual string coil tubing into a well bore for a downhole operation.
In accordance with one aspect of the invention, a seal assembly for dual string coil tubing injection into a subterranean well comprises a seal plate adapted to be connected to a wellhead. The seal plate has a top surface, a bottom surface and first and second bores extending through the seal plate between the top and bottom surfaces. The first bore receives a first annular seal adapted to slidingly and sealingly surround a first coil tubing string extending therethrough, and the second bore receives a second annular seal adapted to slidingly and sealingly surround a second coil tubing string extending therethrough. Passages are provided for directing lubricating fluid to the first and second annular seals to lubricate the respective first and second coil tubing strings while the respective first and second coil tubings are injected into and extracted from the wellhead.
The seal plate includes means for mounting the seal assembly directly to a top of the wellhead, or for mounting the seal assembly to a lubricator that is connected to a top of the wellhead. The seal plate includes grooves for positioning an annular seal between the top end of the lubricator or the top end of the wellhead and the bottom surface of the seal plate.
In accordance with another aspect of the invention, a method of preventing fluid leakage during injection of a dual string coil tubing into a subterranean well for downhole operation is provided. The method comprises steps of inserting first and second coil tubing strings through a dual string coil tubing injector and respective annular seals in a seal plate; suspending the seal plate and the first and second coil tubing strings over a wellhead installed on the well; providing a sealed chamber between the seal plate and a closed blind ram of a blowout preventer of the wellhead; opening the blind ram of the blowout preventer; and injecting the first and second coil tubing strings using the dual string coil tubing injector while injecting lubricant to the annular seals in the seal plate.
A downhole tool may be connected to a free end of at least one of the first and second coil tubing strings. If so, the sealed chamber provided in step (c) sealingly contains the downhole tool. The sealed chamber provided in step (c) may be provided with a lubricator respectively connected to a top of the wellhead and the seal plate. When a downhole tool is not required, the sealed chamber provided in step (c) is alternatively provided by sealingly connecting the seal plate to the top of the wellhead while free ends of both first and second coil tubing strings are inserted into the wellhead above closed blind rams of a blowout preventer. A dual bore blowout preventer is preferably provided below the blowout preventer having the blind rams, and the dual bore blowout preventer is closed around the first and second coil tubing strings after the downhole tool or the free ends of the first and second coil tubing strings are inserted downwards past the pipe rams of the dual bore blowout preventer.
The present invention together with the dual string coil tubing injector assembly described in the Applicant""s copending patent application enables downhole operations requiring a downhole tool connected to each of first and second coil tubing strings, or a downhole operation requiring two coil tubing strings serving different functions or serving similar functions at different depths in the well bore.
Other features and advantages of the invention will be better understood with reference to preferred embodiments described below.